The present invention relates generally to laser systems. In particular, an embodiment relates to a fiber laser-based apparatus and a method of operating the same to produce very high peak power, near-infrared, sub-nanosecond laser pulses by passing gated mode-locked pulses through a heavily-saturated fiber amplifier. For example, an embodiment provides fiber laser emission characteristics that are uniquely appropriate for applications that include inspection, illumination, and/or micro-machining. Other embodiments are useful in other applications as well.
Fiber lasers have advanced to become economical and efficient high power infrared laser sources. Average optical output powers of tens of kilowatts are currently available in commercial fiber laser systems. FIG. 1 is a schematic diagram of a conventional fiber laser system with a master oscillator, fiber amplifier (MOFA) architecture. The master oscillator (also known as a seed laser) emits a low power optical signal that is coupled into the amplifier section through an optical isolator. The optical isolator protects the master oscillator from any light counter propagating back through the amplifier section. The amplifier section consists of a length of gain fiber that is pumped by one or more pump lasers (typically diode lasers) through a pump coupler. The gain fiber may be multi- or single spatial mode, polarization random or maintaining, cladding pumped or core pumped, and may have a variety of dopants (for example Yb, Er, Nd, Pr, etc.) depending on the emission and pumping wavelengths. The pump laser light is absorbed by the dopants in the gain fiber, raising the dopants into an excited state. The emission from the master oscillator is amplified through stimulated emission as it interacts with the excited dopants implanted in the fiber core.
An often used source for short pulse durations (<1 ns) is a mode-locked laser, where the pulse repetition rate of a mode-locked laser is fixed and has a single longitudinal gain mode. In order to decrease the repetition rate, pulse selection has been used in the past. Pulse selection requires a “pulse-picking” apparatus (amplitude modulator, acousto-optic deflector, etc.) that is precisely synchronized with the master oscillator pulse train using a phase-locked loop. In this way, the repetition rate may be changed to any sub-harmonic of the starting repetition rate. While this method is functional, the precise synchronization and requirement for a short selection window (less than one pulse period) are complex and difficult to achieve.
Many variants of the above design are used, including but not limited to the use of multiple gain stages with multiple pumps, the inclusion of various filtering elements, a delivery fiber at the output of the laser, and forward and/or backward propagating pumps. Fiber lasers can operate with a wide range of output parameters to satisfy the varying constraints of an application. It is the specifications of the individual fiber amplifier subsystems that determine the output emission. The output emission of a fiber laser can be specified with the average output optical power, peak output optical power, temporal pulse width, center optical wavelength, polarization, spatial mode, and spectral bandwidth. Pumping limitations, gain limitations, optical damage to components, and nonlinear impairments require a unique system design of the elemental blocks of a fiber laser to achieve the desired set of output parameters.
Fiber lasers are of particular interest as an efficient and compact pulsed source for nonlinear frequency conversion from near infrared (NIR) to visible wavelengths. Nonlinear frequency conversion requires high peak power, narrow optical bandwidth, linear polarization, and single spatial mode. However, it has not yet been practical to simultaneously satisfy these requirements in a pulsed fiber lasers due to nonlinear impairments. In particular Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS), and Self Phase Modulation (SPM) limit the performance of fiber lasers. These nonlinear impairments increase with higher peak intensity in the fiber, with narrower spectral bandwidth, and by propagating linear polarized light. Examples of mode-locked fiber lasers are known in the art that reduce nonlinear impairments because of the large natural bandwidth of the femtosecond (fs) pulses they create, as well as nanosecond (ns) pulsed fiber lasers that use a master oscillator with artificially high optical bandwidth to reduce nonlinearities. Similarly, fiber lasers with kilowatt (kW) average power are known in the art. These lasers function in continuous wave operation, and generally do not approach the 20-500 kW peak powers appropriate for some applications.
Thus, there is a need in the art for improved methods and systems related to high power mode-locked fiber lasers.